Four of Africa's 11 vulture species have been listed as critically endangered this year on an international "red list" of species under threat, a conservation group has said.

BirdLife International said the species are the hooded vulture, the white-backed vulture, the white-headed vulture and Ruppell's vulture.

The status of another two species, the Cape vulture and the lappet-faced vulture, has also dropped from vulnerable to endangered, the group said.

African vultures face a number of dangers, such as feeding on carcasses poisoned by livestock herders to kill predators on the ground, according to BirdLife International.

Many are poisoned by poachers who fear the presence of vultures will alert authorities to the carcasses of illegally killed wildlife, or are killed for their body parts, which are used in traditional medicine.

Habitat loss as well as collisions with wind turbines and electricity pylons are also contributing to vulture population declines.

Mark Anderson, a vulture expert in South Africa, said vultures clean up the landscape by feeding on carcasses and also reduce disease among animals.

The digestive tracts of vultures are so acidic that they destroy anthrax spores that would otherwise spread easily, he said.